The present invention relates to optical communications systems and, more particularly, to an optical star repeater for an optical star network which allows a plurality of remote terminals to communicate with each other.
Optical communications systems have recently made remarkable progress and have been extensively used in a variety of fields. One such application is an optical network in which a plurality of remote communications terminals are interconnected by an optical star network to perform multiplex, time-division transmission and reception of optical signals. This type of optical network, or optical star network as it is generally referred to, has various advantages. For example, system flexibility and economy are excellent, trouble due to electromagnetic induction are eliminated since transmission paths are electrically non-conductive, and circuit reliability is high since only the communications terminals which are transmitting or receiving are required to remain in operation.
A typical optical star network of the type described above is schematically shown in FIG. 1. Optical signals transmitted from various optical communications terminals, or transceivers, 201-20n reach a star coupler 101 via optical transmission paths 311-31n, respectively. The star coupler 101 distributes each of the incoming optical signals to the transmission paths 311-31n, thereby sending each incoming signal to each communications terminals 201-20n. Such an optical star network is described in Eric G. Rawson and Robert M. Mecalte "Fibernet: Multimode Optical Fibers for Local Computer Networks", IEEE Transactions on Communications, Vol. COM-26, No. 7, July 1978, pp. 983-990 (Rawson et al.).
For such a network control, and in the case of electrical communications, a control system of the type which monitors the presence or absence of a received signal and the presence or absence of data collisions is usually used to detect a free time for transmission.
However, where numerous optical communications terminals are interconnected by means of the above-described optical star network, a star coupler which simply comprises an optical branching device would result in critical losses. It is therefore advantageous to use an optical star repeater so that a received optical signal may be converted into an electrical signal and, after regeneration, into an optical signal to be retransmitted. Such a device is disclosed in FIG. 4 of Rawson et al. The received optical signal, unlike an electrical signal, has an intensity which differs from one communications terminal to another. Therefore, should optical signals from various communications terminals be received together, optical signals of high intensity may conceal those of less intensity during regneration. For this reason, in contrast to an electrical communications network, an optical star network is incapable of performing communications controls by simply having each transmission terminal monitor data collisions.
In light of this, a network has been proposed which uses an optical star repeater of the type illustrated in FIG. 2 in place of the optical star coupler shown in FIG. 1 in order to regenerate and redistribute optical signals. In operation, the network of FIG. 2 receives and regenerates optical signals coming from different optical communications terminals by means of discrete optical receivers 111-11n. The regenerated optical signals are routed via line transmitters 121-12n to a bus line 131 which operates in a manner similar to a conventional electrical data bus. The optical signals then pass to a line receiver 141 and are transmitted as optical signals from an optical transmitter 151. This type of optical star repeater is described by Eric G. Rawson et al. in "Fibernet II: an Active Star-Configurd FIber-Optic Local Computer Network with Data Collision Sending", Technical Digest, Topical Meeting on Optical Fiber Communication, pp. 22-23, Apr. 1-15, 1982. A drawback encountered with such a network is that the star repeater has to be furnished with the same number of optical receivers as the optical communications terminals, resulting in an expensive and bulky construction.